46 



NATURE 



[January 12, 1922 



1 12 1, showing the depths and the course of the 

 isohalines. It will be seen that unmixed Atlantic 

 water, of a salinity less than 36-5 per mille, flows 

 in along the north coast of Africa. Midway 

 (Station 1120) we find a slight, and farther north, 

 off the coast of Spain (Station 1121), a somewhat 

 more pronounced, admixture of Mediterranean water. 

 From this it must be concluded that the velocity 

 of the east-going current is at its highest close in 

 to the African shore, and lowest off the coast of 

 Spain, which is also in accordance with fact. The 

 section further shows that the inflow of Atlantic 

 water is a comparati\-ely superficial phenomenon, 

 almost pure Mediterranean water being found at a 

 relatively slight depth. 



For nearly a week at the beginning of October 

 the Dana remained at Gibraltar in order to study 

 the inflow^ of Atlantic water and the accompanying 

 migration of pelagic organisms through the Straits. 

 On several occasions continuous investigations were 



De.pt. 



aoM. 



S.10.21. 9.10.21. 



/2«r,. 3 am. 6 



1010.21 

 3a.m. 



300 



Fig. 3.— Straits of Gibraltar: St. 1138, lat. 35 59' N., long, 

 observations at the same spot from 9 p.m. October 8 to 6 a.m 

 periodical shifting of the level at which salinities of 37-4 and 



temperature of 14° C. are found during the period of observation. "The depths are given 

 in metres. 



made, as, for instance, from 9 p.m. on October 8 

 to 6 a.m. on October 10, when series of water 

 samples, with observations of temperature, w^ere 

 taken every three hours; in the intervals between 

 hydrographical observations fishing with pelagic 

 nets was carried out at various depths. All these 

 investigations were made at the same spot, about 

 the middle of the Straits, w-ith Tarifa, in Spain, to 

 the west-north-west. The investigations showed 

 that both physical and biological conditions varied 

 greatly in the course of the twenty-four hours. 

 Temperature and salinity, for instance, did not 

 remain constant at a given depth throughout that 

 time. Fig. 3 shows how water of 14° C. and 

 37-4 per mille salinity — characteristic values for 

 temperature and salinity of mixed Atlantic and 

 Mediterranean water — changes its level within the 

 twenty-four hours. It will at once be noticed that 

 the changes are periodical, and a closer investiga- 

 tion of the times indicated places it beyond doubt 

 NO. 2724, VOL. 109] 



that we have here to deal with a tidal phenomenon,, 

 albeit the magnitude of the alteration in level would 

 appear to be also dependent upon other factors^ 

 such as the direction and force of the wind. 



A comparison of the contents of the pelagic nets 

 throughout the night at our station oft' Tarifa 

 proved highly interesting, but raised questions 

 which I soon perceived were to be solved only by 

 a far longer stay at this point than the Dana could 

 afford. I will take one example. A net drawn 

 horizontally at about 150 metres' depth on 

 October 8 for two hours from 10.15 p.m. brought 

 up about 80 litres of matter, consisting almost 

 exclusively of a siphonophore (Diphyes). Towards 

 morning Diphyes had disappeared, the contents of 

 the net from 150 metres' depth then consisting 

 chiefly of Salpse and a schizopodous crustacean. 

 The same phenomenon was repeated next night : 

 first scarcely anything but Diphyes, and later on 

 almost exclusively Salpse. Our hauls thus showed 

 that the depth at which Diphyes 

 and Salpoe are found is subject to 

 ' alteration, but the question then 



arises whether such alteration is 

 due to active movement of the 

 organisms in question, or to a 

 purely passive shifting answ-ering 

 37.^%« to the change in level of the water 

 "*° ° layers effected by the action of the 

 tides. 



A comparison of the pelagic 

 fauna on both sides of the Straits 

 of Gibraltar was likewise of much 

 interest. As the main result it 

 ^®'^° may be stated that several species 

 were common to both areas, w-hile 

 others were found only west of 

 Gibraltar. Among the latter may 

 be quoted, of fishes : Argyropdc- 

 cus Olfersi, Vinciguerria Sanzoi, 

 5' 30' w. Continual Myctoflium latematum, and M. 



October lo, showing ^, ■'.,,.'. . , . -j 



-,8-o per mille and a I aldivia, With various mursenoiu 

 larva; {Leptocep]ialus Synaflw- 

 hranchi pinnati, L. latissimus, 

 L. lanceolatoidcs, etc.). In contrast to these 

 1 may mention the larva; of the common 

 freshwater eel {Leptocephalus brevirostris), which 

 pass through the Straits of Gibraltar in enormous 

 numbers. This phenomenon was one of particular 

 interest to the expedition, and I hope to be able 

 to say more about this to readers of Nature later 

 on. Here again we are faced with new problems. 

 Why, for instance, should certain pelagic species 

 stop just outside the Straits of Gibraltar, and 

 others, which out here may be taken in the same 

 haul with the former, not be found in the Medi- 

 terranean? Are they killed immediately on 

 entering the Mediterranean by the natural con- 

 ditions prevailing there, or are they able in some 

 w^ay, despite their pelagic habit, to maintain their 

 position — possibly by means of vertical migrations? 

 Various features w^ould seem to suggest that it is 

 not sufficient to regard the problem solely from the 

 point of view of direction of current as found by 



